This single-arm, open-label study will evaluate the safety and efficacy of the combination oxaliplatin, capecitabine, and trastuzumab with chemoradiotherapy in the adjuvant setting in participants with curatively resected HER2+ gastric or gastroesophageal junction cancer. Participants will receive trastuzumab 8 milligrams per kilogram (mg/kg) intravenously (IV) on Day 1 of Cycle 1 and 6 mg/kg IV on Day 1 of every following 3-week cycle, with oxaliplatin 100 milligrams per square meter (mg/m^2) IV on Day 1 of Cycles 1-3, and capecitabine 850 mg/m^2 orally twice daily on Days 1-14 of Cycles 1-3 and on 5 days per week during chemoradiotherapy. Radiotherapy will be given at a total dose of 45 gray (Gy) divided into 25 doses on 5 treatment days each week for 5 weeks starting Day 22 of Cycle 3. Anticipated time on study treatment is 1 year plus a 1-year follow-up period.